Which of the following is true of the intelligence based on adoption and twin studies

What role do genetic and environmental influences play in determining intelligence? This question has been one of the most controversial topics throughout the history of psychology and remains a hot topic of debate to this day.

In addition to disagreements about the basic nature of intelligence, psychologists have spent a great amount of time and energy debating the various influences on individual intelligence. The debate focuses on one of the major questions in psychology: Which is more important—nature or nurture?

Do Genetics or Intelligence Play a Bigger Role in Determining Intelligence?

Today, psychologists recognize that both genetics and the environment play a role in determining intelligence.

It now becomes a matter of determining exactly how much of an influence each factor has.

Twin studies suggest that the variance in IQ is linked to genetics. This research suggests that genetics may play a larger role than environmental factors in determining individual IQ.

One important thing to note about the genetics of intelligence is that it is not controlled by a single "intelligence gene." Instead, it is the result of complex interactions between many genes. Next, it is important to note that genetics and the environment interact to determine exactly how inherited genes are expressed.

For example, if a person has tall parents, it is likely that the individual will also grow to be tall. However, the exact height the person reaches can be influenced by environmental factors such as nutrition and disease.

A child may be born with genes for brightness, but if that child grows up in a deprived environment where he is malnourished and lacks access to educational opportunities, he may not score well on measures of IQ.

Evidence of Genetic Influences on Intelligence

• Studies show that IQ scores of identical twins may be more similar than those of fraternal twins.
• Siblings who were raised together in the same environment have more similar IQs than those of adopted children who were brought up in the same household.
  

In addition to inherited characteristics, other biological factors such as maternal age, prenatal exposure to harmful substances, and prenatal malnutrition may also influence intelligence.

Studies have found that people with lower intelligence are more likely to report criminal victimization, which can have serious consequences including physical injury, loss of property, and psychological and emotional trauma.

Evidence of Environmental Influences on Intelligence

• Identical twins who were raised separately have less similar IQs than those of identical twins who grew up in the same household.
• School attendance has an impact on IQ scores .
• Children who breastfed for 12 months or longer had a higher IQ (about 3.7 points) at age 30.

So what are some of the environmental influences that can account for variances in intelligence?

For example, studies have found that first-born children tend to have higher IQs than later-born siblings.

Why? Many experts believe that this is because first-born children receive more attention from parents. Research also suggests that parents expect older children to perform better on a variety of tasks, whereas later-born siblings face lesser task-focused expectations.

Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

1. Plomin R, von Stumm S. The new genetics of intelligence. Nat Rev Genet. 2018;19(3):148-159. doi:10.1038/nrg.2017.104

2. Zheng Y, Rijsdijk F, Arden R. Differential environmental influences on the development of cognitive abilities during childhood. Intelligence. 2018;66:72-78. doi:10.1016/j.intell.2017.11.005

3. Boutwell BB, Connolly EJ, Barbaro N, Shackelford TK, Petkovsek M, Beaver KM. On the genetic and environmental reasons why intelligence correlates with criminal victimization. Intelligence. 2017;62:155-166. doi:10.1016/j.intell.2017.04.003

4. Oommen A. Factors influencing intelligence quotient. JNSK. 2014;1(4). doi:10.15406/jnsk.2014.01.00023

5. Ritchie SJ, Tucker-Drob EM. How much does education improve intelligence? A meta-analysis. Psychol Sci. 2018;29(8):1358-1369. doi:10.1177/0956797618774253

6. Horta BL, Hartwig FP, Victora CG. Breastfeeding and intelligence in adulthood: due to genetic confounding? The Lancet Global Health. 2018;6(12):e1276-e1277. doi:10.1016/S2214-109X(18)30371-1

7. Lehmann J-YK, Nuevo-Chiquero A, Vidal-Fernandez M. The early origins of birth order differences in children’s outcomes and parental behavior. J Human Resources. 2018;53(1):123-156. doi:10.3368/jhr.53.1.0816-8177

By Kendra Cherry
Kendra Cherry, MS, is an author and educational consultant focused on helping students learn about psychology.

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